Papers by Author: Tomaž Kosmač

Abstract: Moderately porous Y-TZP dental ceramics were designed to combine the high strength and fracture toughness of densely sintered Y-TZP and the relatively low elastic modulus of porous Y-TZP ceramics. The materials were produced by biscuit-sintering dry-pressed pellets at different temperatures in an ambient air atmosphere. The flexural strength progressively increased with the fractional density, starting from 150–200 MPa at 55–60 % of theoretical density (TD), and exceeding 500 MPa at >80 % TD. The elastic modulus of the early-stage sintered material was matched with that of dentine, but the flexural strength was too low for a practical application. Strength values equal to or higher than those of the existing dental glass-ceramics and glass-infiltrated ceramic materials were achieved with specimens sintered to 8085 % TD, but the elastic modulus of these ceramics (100130 GPa) was about two times higher than that of dentine. Soft grinding of the biscuit-sintered samples resulted in a noticeable increase in the surface roughness, also leading to a significant strength reduction.

Abstract: This study was designed to evaluate the effects of dental grinding and sandblasting on the ageing and fatigue behavior of pressure less-sintered biomedical grade Y-TZP ceramic. It was found that upon dental grinding and sandblasting, the surface of the material was heavily damaged in part plastically deformed, but the amount of transformed monoclinic zirconia was low. The partitioned tetragonal zirconia grains and pre-existing monoclinic zirconia in the ground and sandblasted surfaces hindered the propagation of the diffusion-controlled transformation during subsequent ageing. Dental grinding at a high rotation speed lowered the mean strength under static loading and the survival rate under cyclic loading. Sandblasting, in contrast, resulted in surface strengthening and substantially higher survival rate under cyclic loading. For all tested groups, about 10-15 % lower survival strength values were obtained when tested in artificial saliva, compared to dry specimens, implying that stress-assisted corrosion plays an important role in the fatigue behavior of dental zirconia.

Abstract: The wear resistances of four standard-grade high-alumina ceramics were evaluated and related to their machining ability. Three of the material grades contained 96% of alumina and 4% of either calcium silicate, or magnesium silicate, or manganese titanate in the starting-powder composition. The nominal alumina content in the fourth material was 99.7%. The specimens were fabricated using a low-pressure injection-molding forming technique, followed by thermal de-binding and sintering. After sintering the four materials differ significantly in their grain size, bending strength and Vickers hardness. No direct relationship between the microstructural parameters and the mechanical properties was found, but there was a grain-size dependence of the surface finish after grinding under industrial conditions. The two silicate-containing ceramics exhibited considerably higher wear resistances than the two silicate-free ceramics, but no direct relationship between the abrasive wear rate during grinding and the cutting time was observed. The cutting ability represents a valuable material characteristic for industrial practice, but it should not be directly used for predicting the wear rate during grinding. Quantitative differences in the cutting time and abrasive wear rate were manifested in the different topographies of the worn surfaces. Cutting resulted in relatively large area fractions of plastically deformed surfaces, whereas pullouts dominated the worn surfaces after grinding.

Abstract: In the present study, interactions between AlN and SiAlON laminated couples were investigated after gas pressure (GPS) and spark plasma sintering (SPS) by scanning electron microscopy (SEM) and energy dispersive x-ray analysis (EDX) with the aim to produce laminated composites. In the laminated couples sintered by GPS, a significant reaction zone (~100-150 μm), containing a high aspect ratio of elongated polytypoid grains, was observed at the interface. However, in the case of laminated couples sintered by SPS, a considerably thin reaction region (~2-3 μm) was observed, elongated polytypoid grain formations were also detected.

Abstract: The aim of this study was to investigate the effect of surface treatments on bonding strength of two kinds of zirconia to two kinds of dental cements. After thermal cycling, the shear bonding strength of both zirconia with alumina coating increased in both cements. The shear bonding strength of both zirconia with sandblasting decrease in Fuji plus after thermal cycling (p<0.05). There was no significant difference between NANOZR and Y-TZP (p>0.05). The bonding strengths of NANOZR with the alumina coating were higher than those with sandblasting after thermal-cycling (p<0.05).

Abstract: The effects of dental grinding and sandblasting on ageing and fatigue behaviour of pressure less sintered biomedical grade Y-TZP ceramics were investigated. Disc-shaped specimens were sintered to high density for subsequent surface treatment and testing. Accelerated aging experiments by autoclaving in artificial saliva were performed under isothermal conditions at 1340C. The amount of monoclinic zirconia in the ground and sandblasted specimens was < 5% and about 15 %, respectively and the corresponding strength values were 920 MPa and 1290 MPa. After autoclaving for 24 hours, the amount of transformed monoclinic zirconia in as-sintered material was increased to 15 %, resulting in about 10% strength reduction. In contrast, no strength reduction was observed with the ground and sandblasted specimens subjected to prolonged ageing. At any ageing condition, the highest survival rate during mechanical fatigue testing was observed with the sandblasted samples and the lowest with the ground samples. The strength of the surviving specimens tested in air corresponded well to the mean flexural strength of the particular group before fatigue testing, whereas lower survival strength values and a larger variability in strength were obtained with specimens that were subjected to prolonged accelerated ageing prior to mechanical fatigue testing in artificial saliva.

Abstract: The reactivity of CaTi4(PO4)6 (CTP) with alumina and yttria-stabilized zirconia (Y-TZP) ceramics was studied. CTP powder was synthesized and composites with commercial alumina or zirconia matrices containing 10 wt% of CTP were prepared. They were sintered at different temperatures and characterized using XRD, SEM, and EDX analyses. The results showed that the alumina/CTP and Y-TZP/CTP composites start to react below 1000 °C. In the alumina/CTP composite the first reaction product, detected at 970 °C, was AlPO4. At temperatures above 1280 °C TiO2 and CaTiO3 were also formed and no CTP peaks could be detected using XRD analysis. The composite sintered at 1500 °C consisted of Al2O3 matrix, AlPO4, TiO2, CaTiO3 and Al2TiO5. The reaction products formed in the Y-TZP/CTP composite at 970 °C were TiO2 and Ca2Zr7O16. At higher sintering temperatures, 1280 °C and above, CTP was no longer present, Ca2Zr7O16 decomposed, forming CaO2 and ZrO2, and Y2O3 was consumed to form YPO4. Consequently, upon cooling to room temperature the matrix phase transformed to monoclinic ZrO2. Based on these results it can be concluded that CTP is not a suitable bioactive second phase for the fabrication of CTP composites with alumina or zirconia matrices.

Abstract: For the production of ceramics containing AlN as a major or minor constituent it is necessary to avoid hydrolysis. To do that, non-aqueous powder processing is required or waterresistant AlN powders must be used. Alternatively, as in the Hydrolysis Assisted Solidification (HAS) process, which exploits the hydrolysis of the AlN for the solidification of the ceramic suspensions, the hydrolysis has to be prevented at room temperature but initiated at elevated temperatures. In this work a systematic study of AlN powder reactivity in water and other aqueous environments is presented. The AlN hydrolysis was investigated by measuring the pH of diluted suspensions and by analysis of the reaction products. The results indicate possible solutions for control of the reaction with water in order to exploit it or to prevent it to enable aqueous AlN powder processing.

Abstract: Apatite-forming ability on a zirconia surface was investigated using a conventional SBF (c-SBF), a 1.4 SBF (concentration of ions 1.4 times higher than in SBF) and a supersaturated Ca/P solution. After 5 days a small amount of hydroxyapatite was formed on the zirconia surface in the 1.4 SBF, whereas no apatite was detected on the zirconia surface exposed to the c-SBF for 5 days. In the supersaturated Ca/P solution some isolated clusters of hydroxyapatite were observed on the zirconia surface after only 2 hours, and after 27 hours of immersion time, the zirconia surface was homogeneously covered with hydroxyapatite. Some of the covered zirconia substrates were heat treated at 1050°C for 1 hour. The thermally treated and untreated samples were immersed in SBF for 7 and 14 days. The results confirmed that additional hydroxyapatite was formed in the SBF in both cases, regardless of whether the covered zirconia substrate was pre-heated or not.

Abstract: A design study for ceramic pistons was undertaken in order to address the overheating problems associated with the caliper during aggressive braking using modern C/C-SiC brake discs. The reasoning behind the piston design and the choice of the shaping technology are explained. Zirconia-toughened mullite (ZTM) ceramic was chosen for the pistons, because this material exhibits a low thermal conductivity, sufficiently high mechanical strength and fracture toughness, as well as very good thermal-shock resistance. The manufacturing technology of experimental ceramic brake pistons involved compounding of the selected starting powders to form a high-solids-loadingcontaining feedstock, tooling, shaping of green parts by low-pressure injection molding, extraction of the organic binder, pressure-less sintering in air and final machining. Prototypes brake pistons were fabricated and successfully tested as components of motorcycle brake systems in a test laboratory and on a motorcycle racing track.